The present invention relates to a driving force distributing apparatus for a four-wheel drive vehicle that is capable of automatically canceling an initial torque given by a pressure type differential limiting clutch at a predetermined running speed.
Since a four-wheel drive vehicle is excellent in rough road running property, and also in acceleration property and running stability on ordinary roads, it has rapidly become popular. Direct connection between front and rear wheels can realize a good driving system of the four-wheel drive vehicle. However, in such a state, while there is a merit that a strong four-wheel driving state can be attained, there is a disadvantage that the turning performance becomes poor. Namely, a difference in rotational speed generated between the front and rear wheels at the time of a turning can not be absorbed, and so-called tight corner braking phenomena occur.
Accordingly, in the four-wheel drive vehicle put to practical use, a suitable driving force distribution device is provided between front and rear wheel transmission shafts. Namely, the front and rear wheels are coupled through the driving force distribution device in the state where a predetermined differential rotation is allowed.
As the driving force distribution device, a center differential, a torque coupling, or the like is often adopted. A viscous coupling or a multiple disk clutch is used as the torque coupling. Further, as the torque coupling, there is a torque coupling of a so-called hydraulic type. In the torque coupling of the hydraulic type, a hydraulic pump is provided in a casing that is interlocked and rotated with a transmission shaft connected to one of front and rear wheels, and a rotor that is accommodated in the casing and is interlocked and rotated with a transmission shaft to the other.
However, when the driving force distribution device is provided between the front and rear wheel drive shafts, a slip becomes apt to occur at one of the front and rear wheels at the time of running on a rough road such as a low xcexc road (in other words, a road having low friction coefficient). Accordingly, the traction control becomes difficult as compared with a direct coupling four-wheel drive vehicle at that time.
Thus, in many cases, the driving force distribution device is preferable to provide with a differential limiting device. In the differential limiting device, an initial torque at such a level that the tight corner breaking phenomenon can be allowed is added between the front and rear wheel transmission shafts.
For example, in Japanese Patent Unexamined Publication No. Hei. 10-44802, a multiple disk clutch is adopted as a differential limiting device. An initial torque is generated by a hydraulic pressure, a load of a coned disk spring for pressing the multiple disk clutch, to thereby obtain a differential limiting torque at the time of start.
It is preferable to give a differential limiting torque to front and rear transmission shafts to improve vehicular starting performance. On the other hand, in the conventional structure in which the initial torque is generated by the multiple-disk clutch using friction engagement elements and the coned-disk spring that applies a load to the friction engagement elements, the initial torque is always generated between the front and rear transmission shafts, even in a region where vehicle speed is high and the initial torque is not needed. This is a problem. Namely, in the case where a differential rotation occurs between the front and rear wheels, the initial torque is generated, so that this initial torque acts as an output loss of a driving system. Accordingly, there are disadvantages that an uncomfortable feeling is given to a driver, and fuel consumption becomes larger.
In order to deal with this, for example, Japanese Patent No. 2970913 discloses a technique for actively controlling the intermittent ON-OFF operation of a multiple disk clutch by using a hydraulic pressure. According to the technique recited in this publication, in a region where the initial torque is not needed, the multiple disk clutch is actively disconnected, so that the differential limiting torque can be cancelled.
However, in the technique disclosed in this publication, a hydraulic pressure source for operating the multiple disk clutch is always needed, and it can not be easily attached (adapted) to a driving force distribution device without the hydraulic pressure source, and there is a problem that flexible controllability is lacking.
In view of the above, the present invention has an object to provide a driving force distribution device for a four-wheel drive vehicle where a hydraulic pressure source is unnecessary, a differential limiting torque can be actively cancelled, and an uncomfortable feeling is not given to a driver, fuel efficiency is improved, and flexible controllability is excellent.
The above-mentioned object can be attained by a driving force distributing apparatus, according to the present invention, comprising:
a differential device provided in a vehicle for torque distribution;
a differential limiting clutch provided to variably apply a differential limiting torque to limit a differential operation of the differential device;
an initial torque applying mechanism provided to apply an initial torque to the differential limiting clutch; and
a torque limiter provided to cancel the initial torque at a predetermined vehicle speed.
In addition, the above-mentioned object can also be achieved by a driving force distributing apparatus for a four-wheel drive vehicle, according to the present invention, comprising:
a center differential device provided distribute a torque from an output shaft of a transmission to a front drive shaft operatively connected to front wheels and a rear drive shaft operatively connected to rear wheels;
a differential limiting clutch provided to variably apply a differential limiting torque to limit a differential operation of the center differential device;
an initial torque applying mechanism provided to apply an initial torque to the differential limiting clutch; and
a torque limiter provided to cancel the initial torque at a predetermined vehicle speed.
In the driving force distributing apparatus, it is advantageous that the torque limiter is provided between the front wheel shaft and the rear wheel shaft.
In the driving force distributing apparatus, it is preferable that the torque limiter is provided between the output shaft and the front wheel shaft.
In the driving force distributing apparatus, the torque limiter may be provided between the output shaft and the rear wheel shaft.
Further, the above-mentioned object can also be attained by a driving force distributing apparatus for a four-wheel drive vehicle, according to the present invention, comprising:
a center differential device provided to distribute a torque from an output shaft of a transmission to a front drive shaft operatively connected to front wheels and a rear drive shaft operatively connected to rear wheels;
a differential limiting clutch provided to variably apply a differential limiting torque to limit a differential operation of the center differential device, the differential limiting clutch having a clutch drum operatively connected to one of the output shaft, the front drive shaft and the rear drive shaft, a clutch hub operatively connected to the other one of the output shaft, the front drive shaft and the rear drive shaft, a plurality of plates interposed between the clutch drum and the clutch hub, and a pressure plate operative to press the plates to engage the clutch drum and the clutch hub;
a spring mechanism provided to apply a spring force to the pressure plate to preload the differential limiting, thereby to generate an initial torque; and
a torque limiter provided to apply a force opposite to the spring force depending on vehicle speed, to thereby cancel the initial torque at a predetermined vehicle speed.
In the driving force distributing apparatus, the spring mechanism may be a cone disk spring.
In the driving force distributing apparatus, it is advantageous that the torque limiter comprises a plurality of weights interposed in a space having a triangular-cross-section defined between cam surfaces formed on the clutch hub and the pressure plate, respectively, whereby the weights move in a radius direction depending on vehicle speed and push back the pressure plate against the spring force according to the cam surfaces.
Further, in the driving force distributing apparatus, it is more advantageous that the torque limiter further comprises a position plate disposed in the space and having a plurality of pockets formed in a radial direction for supporting movement of corresponding weights.
In the above-mentioned driving force distributing apparatus, necessary driving force is transmitted via the differential limiting clutch by the differential limiting torque generated from the initial torque of the differential limiting clutch. On the other hand, at the time of running, when the vehicle speed exceeds the predetermined vehicle run speed, the torque limiter device with the vehicle speed sensor operated by the centrifugal force thereof is activated, and the torque limiter device disconnects the driving force and cancels the initial torque.
In this embodiment, preferably, the torque limiter includes pressure surfaces formed between the two shafts or the differentials, a position plate provided between the press surfaces, and a weight provided in a slit-like pocket formed radially therein. In addition, the weight is provided between both the pressure surfaces, and at least one of the press surfaces has a taper shape so as toconver the other pressure surface.